


What Needs Must

by cloud_dcst (greatcloudninja)



Series: TKS TsukaSen Week [5]
Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Alpha Shishiou Tsukasa, Alpha/Beta/Omega Dynamics, Anal Sex, Beta Ooki Taiju, Discord: The Kingdom of Shipping (Dr. STONE), M/M, Mating Cycles/In Heat, Nesting, Omega Ishigami Senkuu, TKS Tsukasen Week - Omegaverse, TKSevents, Tsukasen, tkstsukasenweekjan2021, tsusen, 司千
Language: English
Status: Completed
Published: 2021-01-28
Updated: 2021-01-28
Packaged: 2021-03-13 22:39:56
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,839
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29036346
Author URL: https://archiveofourown.org/users/greatcloudninja/pseuds/cloud_dcst
Summary: Senku had never been one to bother making a nest for his heats, preferring to take suppressants and use scent blockers so he could continue going to class and working on his projects.There is just one problem: in the stone world, suppressants and scent blockers no longer exist.Fortunately, Senku and Taiju recently revived alpha Tsukasa. He offers to build a proper nest for Senku's upcoming heat. And when Senku sees the result, he can't help but invite the alpha to join him in it for his heat.
Relationships: Ishigami Senkuu/Shishiou Tsukasa
Series: TKS TsukaSen Week [5]
Series URL: https://archiveofourown.org/series/2123469
Comments: 2
Kudos: 145
Collections: TsukaSen Week January 2021





	What Needs Must

**Author's Note:**

> This is my entry for Day 5 of [TKS TsukaSen Week](https://twitter.com/tksDCSTevents)! I used prompt "Omegaverse" for this fic!

“Why haven’t you built yourself a nest yet?” Tsukasa asked.

Senku looked up from where he was working on making… something. Tsukasa thought it might be more crushed and fire-baked seashells, but he wasn’t entirely sure. The ways of the young scientist were vague and mysterious sometimes.

But the smell of pre-heat coming off of him in waves was a tale as old as time. And Tsukasa didn’t see any sign of an impending nest. No new furs had been added to his pile, despite the chilly early spring air. He just had his one, plain sleeping sack that he dutifully unstitched and aired out every few days.

“What do you mean? My nest is just fine the way it is. I built it when I had my first heat in the stone world and it’s served me well up to this point.”

Tsukasa blinked.  _ That _ was his nest? That shoddy pile of—probably whatever his first kills had been, once he was able to snare and trap mammals? Most of them were so worn and had been badly tanned, to the point where they were practically falling apart.

Then again, omegas were known for being sentimental. It might not be  _ comfortable, _ but it was  _ his, _ and that was what mattered to Senku. Even if he seemed like the least sentimental person Tsukasa had ever met, omega or not. 

“This is how Senku’s always been!” Taiju piped up. As a beta, he was blessedly nose-blind to the pheromones Senku was putting out in waves. “Even when we were younger, he’d pop suppressants, slap a billion scent blocker patches on his wrists and neck, and keep coming to school even through his heats.”

“That’s because if I didn’t, you were liable to fail a test, you big oaf,” Senku muttered. “Besides, heats and ruts are a relic of the past. Mankind hardly had a need for the instinct to breed, whatever the old government said about Japan’s declining birth rates. Science was barely able to sprint to keep up with worldwide population demands.” 

“All that is a very interesting way of saying you were bad at nest-making,” Tsukasa replied; his tone was mild, but the observation was serious. An omega who was unhappy with their nest would be more likely to want to avoid it. And from everything Tsukasa could infer about Senku, he’d never had anyone show him the proper way to build one. “Do you mind if I make one for you?”

Looking up at Tsukasa over his shoulder, Senku’s face was nonplussed. “If you want, be my guest. I don’t give a crap what you do as long as you keep up your end of the bargain with the food and protection.”

“Alright, then. Let me see what I can do.”

They didn’t have a lot of spare furs, but Tsukasa had hunted enough game—both large and small—that they had a decent enough selection, if he shuffled things around between their piles of bedding. Taiju was more than happy to let Tsukasa sort through them all and pick out the best, softest furs for Senku’s new nest. He even crafted a basic pillow out of dried grasses and bird feathers; something that Senku would be able to lean against or hold to his chest if he so wished. 

And if Tsukasa carefully scented every fur and the special pillow… well, Taiju would never notice the difference. The beta would stay upstairs with Senku while he rode out his heat, making sure to keep Senku fed and hydrated, while Tsukasa retreated to the lab to give the omega some privacy.

It took Tsukasa the better part of the day to do his work; but by nightfall, one corner of the elevated hut had been transformed into a soft, inviting pallet for their resident omega. Senku finally retreated from where he’d holed himself up in the laboratory. Tsukasa could smell it on him—his heat had finally arrived in full. 

“So, let’s see this nest you’ve made…” Senku’s feet were a little unsteady under him and he missed one of the rungs of the ladder, but he managed to make his way up unscathed. Tsukasa followed behind and stood in the doorway as Senku stepped into the hut. 

Taiju’s own pallet of furs was much smaller by comparison, and Tsukasa’s was nonexistent—he had gathered up his own just outside and was waiting for Senku to settle down before he went to make his own bed. “Geez, you’ve made it way too big,” Senku observed. “ _ You _ could probably fit in here with me…” 

But Senku seemed pleased as he knelt down in the center of the pile and picked up the pillow. “Oh. This is… this is actually… quite nice.” His fingers trailed over the well-tanned deer hide, buttery soft to the touch and squishy with the feathers and the grasses inside. Setting the pillow aside, he ran his fingers over the collection of furs that had been gathered and tucked around a large pile of more dried grass. The grass provided additional insulation against the cold wooden floor of the elevated platform, and the layered furs made for interesting combinations of textures at Senku’s fingertips. 

Senku’s breaths slowed, but deepened as he took in the invisible marks that Tsukasa had left behind on the nest. “You scented everything, didn’t you?” he asked quietly.

“I did. I thought having alpha pheromones in your nest might help you with your heat,” he replied. “At least, that was how I understood things back in the old days. We’re the closest thing to a pack in this stone world, right?”

Senku hummed in agreement. “True…” 

“So, how do you like it compared to your old nest?” 

“It’s… really nice,” Senku breathed. “You did good. Ten billion points.”

“I’m glad to hear it. I’ll leave you to—”

“Wait… something’s missing.” Senku’s voice cut through Tsukasa’s thoughts like a freshly sharpened stone knife through yielding flesh. “You went through all this trouble… you even made it bigger than just what I needed.”

“Senku… you’re not in a state to consent to this.” 

“What, do you think I’m going to cry ‘rape’ or something afterward? I’m not that type of omega, not one millimeter. But we don’t have time for me to deal with a full week’s worth of heat. A few good knots will speed things along nicely.”

Tsukasa opened his mouth to argue; but he could see the wisdom in Senku’s words. “And if you get pregnant?”

“You think I haven’t already been planning for that?” Senku asked. “Honestly, I’d been planning to ask you to help me through this ever since we revived you. I knew my heat would be coming soon and I’ve been collecting some special herbs for this exact reason. Obviously, no birth control method is 100% safe, but this is as good as we’re going to get in the stone world short of making condoms from animal intestines.”

Tsukasa pulled a face. He vaguely remembered reading once that those were the norm at one time, but the idea of trying to stuff his knot into an intestine like some kind of sausage was less than appealing. “Fair enough. If you’re sure you’re okay with it, then I’ll be happy to help you.”

“It just makes sense.” Senku’s voice was strangely withdrawn, as if he was trying to convince himself more so than Tsukasa. “The sooner I finish my heat, the sooner we can get back to work.”

“Alright.” Sticking his head back out the door, Tsukasa called down to Taiju, “Sorry, friend, but it looks like Senku and I will be spending the next few days up here together… will you be alright by yourself?”

“Sure thing, buddy! I already stocked plenty of food and fresh water up there for Senku! Just toss me my furs and I’ll make my own cozy little nest down here in the lab!” Tsukasa nodded and did just that; adding his own furs to the mix for Taiju to use for additional warmth. By the time he returned inside the hut, Senku had already stripped out of his roughly stitched tunic and was leaning back with his head on the pillow Tsukasa had made. His under-skirt was still tucked into place, though, tantalizing the alpha with the promise of what lay underneath. 

Tsukasa could smell the slick already; a honey-sweet scent laced with a minty, herbaceous note that matched Senku’s natural pheromones. He had to bite down on his tongue to hold back a growl at the way his cock jumped to attention. Never in his life had Tsukasa been so close to an omega in full heat—not like  _ this, _ anyway. He tried to take a deep breath to calm himself… but only succeeded in taking on a full nose of heat-slicked, pheromone-soaked air. 

“What the hell’s taking so long?” Senku groused. “Do I need to go into full presentation mode for you or something?” Before Tsukasa could reply, the omega did just that—rolling onto his stomach, he lifted his hips into the air. His skirt hiked up to reveal his dripping hole, slick already leaking out of him. The pucker twitched in the cool air and a high, soft, almost wounded sound escaped Senku’s lips.

The tenuous dam of Tsukasa’s self-control couldn’t stand up to such a brazen display. He fell into the nest on his knees, shucking off his lion pelt and unwrapping his own tunic with fevered movements. “I’m here, Senku,” he murmured. “Sorry for making you wait.”

“Nnn… make it up to me by giving me your knot,” Senku mumbled. He pressed his chest into the pillow under him and spread his legs wider, giving the alpha easier access to tug the under-skirt off completely.

Senku’s completely nude body splayed out under Tsukasa like a feast. Part of him wanted to dive face-first, to lap and suck at the slick that dribbled out of Senku’s hole and drink his fill; but time was of the essence, and the whole point of Tsukasa’s presence was to provide that which Senku’s instincts craved. 

He wrapped his hands around Senku’s hip to steady the smaller body before pressing the head of his cock to Senku’s hole. There was no preparation; only the dripping slick to ease the way. But other than a sharp intake of breath, Senku didn’t seem to react negatively to being stretched open around Tsukasa’s alpha cock. He pressed inside, not stopping until the head had completely breached Senku’s hot, tight hole.

Below him, Senku’s body shuddered.  _ So big! _ Senku hadn’t exactly been experienced with other alphas prior to being petrified, but he’d used heat toys before—he thought he’d known what he was getting into. But Tsukasa was in a class all his own. Barely the head inside and he was already gasping for breath. “W-wait…” The word ripped out of Senku’s mouth with a tiny sob.

Tsukasa froze instantly. “What’s the situation?” he murmured. Had he hurt Senku, pushed him too far already?

“C-can you… f-fingers…” Senku spread his knees a little farther, arched his back just a hair more; lifting his body to try to get a better angle. Tsukasa breathed a sigh of relief and nodded. He pulled back until his cock head popped out, replacing it with a single finger. A soft, broken moan escaped Senku’s lips at the new stretch, but it wasn’t pained. 

This was better. Yes, they were on something of a deadline—every day spent here was one day they couldn’t hunt and gather and build—but they didn’t need to rush. They could take their time; Tsukasa could make it good for Senku. He worked his finger in and out, slowly, one knuckle at a time until the whole digit disappeared inside Senku’s tight, wet hole. Then a second, still at the methodical pace, drawing mewling whines from Senku’s lips.

Tsukasa twisted his fingers around, seeking out that one special spot that would make Senku  _ really  _ see stars. A sharp yelp let him know he found it; Senku’s hole spasmed, clenching around the fingers inside as a dribble of precum escaped his flushed erection. “Th-there!” Senku cried out. 

Moments later, Tsukasa drove three fingers straight inside, hitting Senku’s prostate with precision, and he  _ screamed _ as he came over the furs below him. The omega’s body twitched as Tsukasa pet his flank. An indulgent smile crossed the alpha’s lips. “Good, good omega,” he murmured. “Do you think you can take my cock now?”

_ “Yes, please, alpha!”  _ Senku’s voice was high, tight and reedy—no snark, no need to cover his true emotions. “Need your cock, need your knot!”

“I’ll give it to you.” Tsukasa licked over the back of Senku’s neck, tasting his scent from the source. It was woody, almost minty; not overly flowery or feminine like the handful of omegas Tsukasa had ever had the opportunity to smell up close. He knew better than to bite down—luckily his rut was months away, so he was well in his right mind—but even soft nibbles and licks would satisfy Senku’s instincts. 

Tsukasa braced Senku’s hips with one hand and used the other to lead his heavy erection back to the stretched hole. The second try went much more smoothly, Tsukasa’s head popping inside with little resistance. Leaning over Senku’s body, Tsukasa could feel soft vibrations coming from his back.

Senku was  _ purring  _ as Tsukasa fucked into him. “Does it feel good?” he murmured against Senku’s ear playfully before giving the lobe a nip.

_ “Yes!” _ Squirming under the alpha’s heavy body, Senku rocked back against Tsukasa as best he could, urging the cock to slide deeper into him. “Please, Ts’kaaasa, fill me up… so empty…”

“What do you want?” Tsukasa couldn’t help himself. Senku was so… almost  _ cute _ like this, sweaty and whining though he was. Maybe it was an alpha instinct, or maybe it was just  _ Senku, _ but Tsukasa wanted to tease him a little. 

An errant thought crossed his mind. Senku wanted to revive all seven billion people on the planet… but if Tsukasa claimed him, here and now—if Senku became  _ his _ omega… Tsukasa could…

_ No. _ That was unconscionable. It was one thing to destroy a dead statue. Those people… they were no longer flesh and bone, but stone through and through, no matter what kind of miraculous fluid Senku could concoct. But to force Senku into a mating bite when he couldn’t consent would make Tsukasa nothing more than a monster.

Another high-pitched whine shook Tsukasa out of his thoughts. “Fuck me, Tsukasa… fill me up, knot me, please…” 

Tsukasa nodded. “Of course. Whatever you need.” It wouldn’t take long, anyway; Senku was so hot, so tight, deliciously squeezing around him. The base of Tsukasa’s cock tingled in a way that was familiar, yet entirely new. He’d never felt this kind of pressure, of  _ pleasure _ before. Not with a rut toy, not with his own hand. Nothing could compare to a real omega’s hole.

Or, again, was it just…  _ Senku? _

There was no need to think about it too deeply. He was getting close. Soon he would fulfill his instincts and those of the omega below him. Each thrust was already getting harder; his knot thickening and catching on Senku’s rim. “Ts’kasa, p-please, please—”

“I’m here. I’ve got you.” It was child’s play for Tsukasa to lift Senku bodily into the air, letting gravity assist with forcing his ballooning knot deeper inside. Senku’s arms flew out behind him, scrabbling at Tsukasa’s neck and shoulders to brace himself. But Tsukasa held him in a secure grip and fucked up into him with a series of wet  _ pops. _

Finally, with a broken wail, Senku spilled over his abdomen and clamped down hard around the base of Tsukasa’s knot. It was enough to lock him inside, his own seed forcing its way into Senku’s hole and filling up every available crevice. When Tsukasa looked down over Senku’s body, he could see a faint hint of a bulge in his lower stomach area. He carefully shifted his grip to hold Senku with one arm, then brushed fingers over it—feeling himself through Senku’s stretched skin. 

The touch drew a contented sigh from Senku’s lips, followed by an almost manic chuckle. “So full… Tsukasa is so good… good alpha…”

“You’re an even better omega,” Tsukasa murmured, kissing the back of his neck once more. Senku’s hair had wilted with sweat and heat, hanging down around his head, and Tsukasa buried his face in the surprisingly soft locks. “Let’s rest now. I’ll take care of you.”

“Yeah… okay.” Senku yawned readily enough as Tsukasa carefully lowered them into the nest. They settled on their sides, Senku using Tsukasa’s bicep as a pillow in lieu of the stuffed one.

He was asleep within minutes. Tsukasa traced nonsense lines across his stomach and sighed gently against his hair. 

It was going to be a long few days… Tsukasa just hoped he made it out with his convictions intact.

**Author's Note:**

> Feel free to join the Dr. Stone shipping server! All ships are welcome!  
> We have lots of events scheduled, RP our fave ships, share headcanons, etc!  
> [Click here to join us!](https://discord.gg/RGy78Kj)


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